Interesting stuff indeed! It's amazing what can be picked up reading this kind of discussion, including answers to questions I didn't know needed asking.:-)

I've been testing a few things, including some footage from the only project I ever completed in 50p. I put some raw camera clips on the timeline as Noa suggested, colour corrected and rendered 25p as ProRes. Apart from the jerkiness of motion, I saw an increase in IQ.

Then I tried some 25p footage from my current project, which has been transcoded as always. Again I put some raw camera files on the timeline, same MO as before and again saw an increase in IQ when comparing ProRes renders of each. That's the opposite of what I expected! Methinks I shall save the time and effort of transcoding in future and, you guessed it, start the project again using raw clips. Of course, not transcoding removes the need for my original question, since the camera bitrate will go straight on the timeline.